The invention relates generally to suspension locking of work vehicles. More particularly it relates to automatic suspension locking of skid steer vehicles when the vehicle experiences fore-and-aft acceleration or deceleration beyond predetermined limits.
Skid steer loaders are small highly maneuverable vehicles that are used in place of front end loaders, backhoes and the like in constricted environments. They are particularly useful due to their small size and maneuverability.
Their maneuverability is due to their method of steering. The wheels on one side of the vehicle (typically two per side) can be driven independently of the wheels on the other side of the vehicle. The wheels themselves are not steerable. In other words, they cannot be turned about a generally vertical axis with respect to the chassis.
To steer a skid steer vehicle, the wheels on one side of the vehicle are driven at a different speed than the wheels on the other side of the vehicle. In an extreme case they are also driven in opposite directions. Thus, the wheels on one side can be driven forward as the wheels on the other side of the chassis are driven in reverse.
These different velocities cause the wheels to skid sideways as they rotate. As a result, one side of the vehicle advances faster than the other, and the vehicle turns. In the extreme case, when the wheels on either side are driven in opposite directions, the vehicle can rotate about a vertical axis within the perimeter of the vehicle""s chassis thereby giving it a turning radius of zero.
In addition to the maneuverability provided by the steering arrangement, skid steer vehicles also benefit from a short wheelbase and narrow width. A short wheelbase and narrow width permits the vehicle to be used in confined spaces, but prevents the use of sprung suspensions.
The short wheel base, however, causes poor ride quality of the skid steer and limits the transportation roading speed. A skid steer vehicle tends to bounce, roll, and pitch on rough terrain. A sprung suspension would improve the ride and the stability.
A sprung suspension used on a short wheelbase vehicle, however, would cause excessive diving (due to vehicle deceleration in the direction of travel) and pitching (due to vehicle acceleration in the direction of travel) as the operator slows down or speeds up the motors driving the wheels. Such pitching or diving could cause the suspensions to be compressed to the extent that the contents of the bucket are spilled or the bottom of the vehicle contacts the ground.
For these reasons, skid steer vehicles having a short wheelbase (such as the common skid steer loader) have never been equipped with sprung suspensions.
What is needed, therefore is a suspension system for a skid steer loader that provides independent springing while the skid steer loader is being driven at a steady speed across a work site yet will also reduce or eliminate that springing when the skid steer loader is accelerated or decelerated by the operator to a degree that would cause undue pitching or diving.
It is an object of this invention to provide such a suspension system for a skid steer vehicle.
It is a further object of this invention to provide such a system that automatically locks the front suspensions when the vehicle is decelerated above a threshold deceleration and locks the rear suspension when the vehicle is accelerated above a threshold acceleration.
In accordance with a first embodiment of the invention, a skid steer vehicle is provided that includes a chassis having a left side and a right side; at least one loader arm pivotally coupled to the chassis to pivot about a substantially horizontal axis; at least one hydraulic cylinder coupled to the at least one loader arm to raise and lower the at least one loader arm with respect to the chassis; an engine coupled to the chassis; first and second variable displacement hydraulic pumps coupled to the engine to provide two separately controllable sources of hydraulic fluid under pressure; four non-steerable and ground-engaging wheels coupled to the chassis to drive the vehicle over the ground, wherein the wheels are disposed two on each side of the chassis in a fore-and-aft relation; four control arms pivotally coupled to the chassis and coupled to the four wheels to permit the wheels to pivot at least in a vertical direction with respect to the chassis; at least two hydraulic motors for driving the wheels wherein at least one motor is driven by fluid from the first pump and in turn drives the wheels on the left side of the chassis and at least another motor is driven by fluid from the second pump and in turn drives the wheels on the right side of the chassis; four hydraulic cylinders, each cylinder operably coupled to one of the wheels to control at least the vertical position of the wheels with respect to the chassis; a means for indicating the fore-and-aft acceleration of the vehicle; and an electronic controller operably coupled to the means for indicating and responsive to the means for indicating to (a) lock two rear wheels of the four wheels when the vehicle experiences a forward acceleration greater than a predetermined forward acceleration, and (b) to lock two front wheels of the four wheels when the vehicle experiences a forward deceleration greater than a predetermined forward deceleration.
The means for indicating may be at least one wheel speed sensor indicating the forward speed of the vehicle and the electronic may be is configured to compare successive speed signals from the at least one speed sensor to determine whether the forward acceleration is greater than the predetermined forward acceleration and to determine whether the forward deceleration is greater than the predetermined forward deceleration. The means for indicating may include memory locations in the electronic controller storing at least two sequential values of a commanded specific displacement of at least one of the first and second pumps and the electronic controller may be configured to compare the at least two sequential values to determine whether the forward acceleration is greater than the predetermined forward acceleration and to determine whether the forward deceleration is greater than the predetermined forward deceleration. At least one of the pumps may be configured to generate sequential signals indicative of sequential actual specific displacements, and the means for indicating may include that pump, and the electronic controller may be configured to compare the sequential signals to determine whether the forward acceleration is greater than the predetermined forward acceleration and to determine whether the forward deceleration is greater than the predetermined forward deceleration. The electronic controller may include a drive controller coupled to and controlling the specific displacement of the two pumps, the drive controller being configured to generate the at least two sequential values of a commanded specific displacement and also configured to save the at least two sequential values of a commanded specific displacement in the memory locations. The electronic controller may include a suspension controller coupled to the drive controller and configured to compare the at least two sequential values of a commanded specific displacement to determine whether the forward acceleration is greater than the predetermined forward acceleration and to determine whether the forward deceleration is greater than the predetermined forward deceleration. The suspension controller may be configured to block fluid flow between a first cylinder of the four hydraulic cylinders and a first hydraulic accumulator, and to block fluid flow between a second cylinder of the four hydraulic cylinders and a second accumulator when the forward acceleration is greater than the predetermined forward acceleration. The suspension controller may be configured to block fluid flow between a third cylinder of the four hydraulic cylinders and a third hydraulic accumulator and to block fluid flow between a fourth cylinder of the four hydraulic cylinders and a fourth accumulator when the forward deceleration is greater than the predetermined forward deceleration. The drive controller and the suspension controller may be coupled together over a serial communication bus. The serial communication bus may be a CAN bus. The wheels may be locked by limiting fluid flow from the four hydraulic cylinders to four associated hydraulic accumulators.